Polyelectrolytes have been blended with poly(vinylidene fluoride) and its copolymers (KYNAR PVDF from Arkema Inc.) to take advantage of the physical, chemical, electrochemical, and transport properties characteristic of both the PVDF and the polyelectrolyte components. Films of these polymer blends are useful as fuel cell membranes, and other applications such as water purification, humidification, and as separators for batteries.
A critical limitation of the current materials is that the desirable proton conductivity performance declines rapidly as the local relative humidity decreases. This has implications for the use of these materials as fuel cell membranes and constant externally applied humidification is required for the material to remain at peak performance. This constraint adds cost and complexity to the overall system.
One means for improving the performance is described in PCT/US10/34830, incorporated herein by reference. The reference describes monomers and resultant (co)polymers containing multiple acid groups per monomer unit. It also incorporates the idea of fluorinated or perfluorinated analogues thereof. The reference further describes the synthesis and use of styrenic-type monomers bearing more than one acid group. The disulfonated styrenic monomer and copolymer, blended with PVDF was cast into membranes that showed a marked improvement in the ion-conductivity and in-cell performance under reduced relative humidity conditions.
The industry is always seeking means for obtaining a further increase the proton conductivity of the polyelectrolyte phase. Besides incorporating more acid groups, ion-conductivity may be enhanced by increasing the acidity of the functional groups on the polyelectrolyte. Surprisingly, a relatively simple and benign method has now been found to incorporate an α,α-difluorosulfonic acid functionality (superacid functionality) into the polyelectrolyte. Additionally, the new synthesis process is robust and useful with a large range of different Grignard reagents, providing a family of new superacid functional molecules, including but not limited to monomers. It is expected that polyelectrolytes bearing this functionality will have enhanced proton conductivity properties over those used in previous generations.
Further, the present invention contemplates a facile, high-yielding and generally-applicable synthetic methodology by which a myriad of compounds containing alpha,alpha′-difluorosulfonic acid functionality can be synthesized. Other methods for the introduction of this superacid functionality involve the use of highly reactive and/or toxic materials, (elemental fluorine, tetrafluoroethylene, SO3, etc.), whereas this procedure is relatively benign.
The few other methodologies available for the introduction of this superacid functionality into molecules, involve multiple transformations using highly reactive and/or toxic reagents (elemental fluorine, tetrafluoroethylene, SO3 gas, etc.). The most auspicious of these being the ring opening of (highly toxic) perfluorinated sultones, a chemistry that is used to synthesize the acidic monomer used in NAFION ionomer membranes, commercialized by E. I. DuPont de Nemours Co. Other processes have been described such as the oxidation of α,α′-difluorothiols or thioacetates, however these routes typically involve multiple steps, scantly available starting materials, and low yields.
The present invention is useful in many fields, including ion-conductive membranes, ion-exchange resins, flocculants, metal adhesives, surfactants, water-soluble pharmaceuticals, and other applications where a very strongly acidic, strongly anionic, and/or strongly hydrophilic material is needed.